Apple's "Handoff" -- one of the key new features of iOS 8 and OS X Yosemite -- will work only on newer Macs that support Bluetooth 4.0 and the Bluetooth LE (low energy) technology.

Apple's "Handoff" -- one of the key new features of iOS 8 and OS X Yosemite -- will work only on newer Macs that support Bluetooth 4.0 and the Bluetooth LE (low energy) technology, according to an Apple engineering manager.

While Apple didn't mention Handoff requirements when it unveiled the feature at its Worldwide Developers Conference (WWDC) earlier this month, a developer-only session there made it clear that Bluetooth LE is necessary.

"This is entirely about the proximity of your devices to your other devices," said Michael Jurewitz, lead engineering project manager for Continuity, during a June 4 developer session at WWDC that Apple has made public as an audio recording.

"We use BTLE [Bluetooth low energy] to actually let devices around you know what they should show in that lower left corner [of the screen, where Handoff notifications appear]," Jurewitz continued. "Which devices that actually show your apps is linked to the devices that you are signed into with the same iCloud account. We know you're signed into this device and we actually use the cloud to BTLE-pair your different devices to each other."

After the pairing, small amounts of data are passed between the devices, presumably also via Bluetooth, although Jurewitz wasn't specific.

Handoff, part of "Continuity," a term Apple's used to describe several features of iOS and OS X, allows users to begin an activity -- writing an email, browsing the Web, creating a document -- and then resume it on another device.

Many analysts identified Continuity in general, Handoff in particular, as one of the most important and interesting enhancements to iOS 8 and Yosemite, the two operating system upgrades slated to ship this fall.

But not all Mac, iPad or iPhone owners will be able to use Handoff, according to Jurewitz's comments.

Because Bluetooth LE was merged with the Bluetooth 4.0 specification in 2010, devices that support the latter spec are LE-ready. Apple has supported Bluetooth 4.0 and Bluetooth LE on the iPhone since 2011's iPhone 4S. Perhaps not coincidentally, that also the oldest iPhone which will run iOS 8, which also includes Handoff capabilities.

On Macs, however, it's a slightly different story. For example, the MacBook Pro notebook line didn't get Bluetooth 4.0 until the mid-2012 models, and the iMac not until late 2012. The MacBook Air, a lighter, thinner laptop, and the sans-screen Mac Mini have had Bluetooth 4.0 since mid-2011.

iPads have supported Bluetooth 4.0 since the March 2012 refresh -- Apple's first Retina-equipped tablet -- so 2011's iPad 2 and 2010's original iPad will not work with Handoff.

Mac owners can determine the age of their machine as Apple tracks it by selecting "About This Mac" from the Apple menu at the far left of the menu bar, then choosing "More Info..." from the ensuing window. The Mac's age will appear under the name of the model, as something like "Retina, 13-inch, Late 2012" for a MacBook Pro laptop put into service in early 2013.

A more precise way to identify Bluetooth 4.0 support is to select "About This Mac" from the Apple menu, click on the "More Info..." button, then on "System Report." Select "Bluetooth" under "Hardware" in the left-hand pane, and then find the item "LMP Version." LMP stands for the "Link Management Protocol" used to set up and manage a Bluetooth connection between two devices.

If the value to the right of "LMP Version" is "0x6" then the Mac supports Bluetooth 4.0 and so can use Handoff when the system is upgraded to Yosemite.

OS X 10.10, aka Yosemite, will run on a majority of current Macs -- by Computerworld's estimate, about 80% of those that went online in May -- but the Bluetooth LE mandate means that not all Yosemite-powered machines will be able to hand off tasks or accept incoming jobs from other Apple devices.

At a minimum, 25 million Macs will support Handoff -- that's the total global sales from the fourth quarter of 2012 through the first quarter of 2014 -- but the number is certainly somewhat higher, since the 25 million doesn't account for five quarters of MacBook Air sales, one quarter of MacBook Pro sales, five quarters of Mac Mini sales, and whatever number Apple has managed to sell of the redesigned Mac Pro.

At WWDC, Apple CEO Tim Cook said that the Mac installed base was approximately 80 million machines, which means that at least 32% of all Macs in service will be able to use Handoff.

It's not clear whether third-party Bluetooth adapters, such as the inexpensive dongles that plug into a USB port, will allow Handoff on older Macs.

OS X Yosemite will launch later this year -- most likely in October -- as a free upgrade.